It is generally desirable to have bandpass filters that do not have degradation in transmission properties when they are used at an angle of incidence to the light path. Filters based on optical interference principles are highly versatile, and may be designed for use throughout the entire optical spectrum.
Multi-cavity filters have been manufactured for more than 40 years. The usual approach of filter designers has been to simply anti-reflect the equal length cavity structures to the substrate and the medium. For this simple structure, use at an angle yields filters that for the two polarization modes exhibit different central wavelengths. These types of filters are described in detail by P. W. Baumeister, in, "Optical Coatings Technology," lecture notes for the five-day short course engineering 190.01 at the Continuing Education Institute, Jun. 16-20, 1986, Chapter 1, pg. 1-38-1-41. To lessen this problem, the need to modify the cavity properties was investigated thoroughly by experts in the thin film field. P. Baumeister, "in a paper entitled Bandpass design-applications to non-normal incidence", in Applied Optics 31, 504-512 (1992) where a standing wave ratio technique to match reflective zones by applying microwave filter synthesis was used. These cavities were adjusted to optimize a single angle. T. Yanagimachi et. al., in a paper entitled "High-performance and highly stable 0.3-nm-full-width-at-half-maximum interference optical filters," Applied Optics 33, 3513-3517 (1994), used a staggered thickness approach that is useful for single cavity filters. However, the wavelengths for each polarization vary as the angle is modified. G. P. Konukhov and E. A. Nesmelov, in a paper entitled "To the theory of a dielectric narrow-band filter," Journal of Applied Spectroscopy, vol. 11. pg. 468-474, (1969) describe using a third material for the half wave layers; this technique works well in theory, but in practice there are limits on the choices available. Furthermore, there also may be long-term stability concerns with the various materials.